Camshafts are used in internal combustion engines in order to actuate the gas exchange valves. The camshaft is fitted in the internal combustion engine in such a manner that cams fitted on it bear against cam followers, for example bucket tappets, drag levers or rocker arms. If the camshaft is caused to rotate, the cams roll off along the cam followers which in turn actuate the gas exchange valves. The position and the shape of the cams therefore define both the opening period and amplitude but also the opening and closing time of the gas exchange valves.
Modern engine concepts are based on the valve drive being of variable configuration. On the one hand, the valve stroke and valve opening period are to be able to be variable until individual cylinders are completely shut down. For this purpose, concepts, such as switchable cam followers or electro-hydraulic or electric valve-actuating means are provided. Furthermore, it has proven advantageous to be able to have an effect on the opening and closing times of the gas exchange valves during the operation of the internal combustion engine. It is likewise desirable to be able to have an effect on the opening and closing times of the inlet and outlet valves separately in order, for example, to be able to set a defined valve overlap in a specific manner. The specific setting of the opening and closing times of the gas exchange valves as a function of the current range of performance characteristics of the engine, for example of the current speed of rotation or the current load, makes it possible to reduce the specific fuel consumption, to have a positive effect on the exhaust behaviour, and to increase the engine efficiency, the maximum torque and the maximum power.
The described variability in controlling the timings of the gas exchange valves is brought about by means of a relative change of the phase position of the camshaft with respect to the crankshaft. In this case, the camshaft is in direct drive connection with the crankshaft generally via a chain drive, belt drive or gear drive. A camshaft adjuster which transmits the torque from the crankshaft to the camshaft is fitted between the chain drive, belt drive or gear drive, which is driven by the crankshaft, and the camshaft. This device is designed in such a manner that the phase position between the crankshaft and camshaft is securely held during the operation of the internal combustion engine and, if desired, the camshaft can be rotated over a certain angular range relative to the crankshaft.
In internal combustion engines having a respective camshaft for the inlet valves and the outlet valves, the said valves can be equipped with a respective camshaft adjuster. As a result, the opening and closing times of the inlet and outlet gas exchange valves can be displaced in time relative to one another and the timing overlaps can be set in a specific manner.
Modern camshaft adjusters are generally seated at the drive end of the camshaft. The said camshaft adjuster comprises a timing gear secured on the crankshaft, an output part secured on the camshaft and an adjusting mechanism transmitting the torque from the timing gear to the output part. The timing gear can be designed as a chain wheel, belt wheel or gear wheel and is connected in a rotationally fixed manner to the crankshaft by means of a chain, a belt or a gear drive. The adjusting mechanism can be operated electrically, hydraulically or pneumatically.
In the case of the hydraulically operated camshaft adjusters, a differentiation is made between “axial piston adjusters” and “rotary piston adjusters”.
In the case of the axial piston adjusters, the timing gear is connected to a piston via a helical toothing. Furthermore, the piston is connected to the output part likewise via a helical toothing. The piston separates a cavity, which is formed by the output part and the timing gear, into two pressure chambers arranged axially with respect to each other. If the one pressure chamber is acted upon by a hydraulic medium while the other pressure chamber is connected to an oil outlet, then the piston is displaced in the axial direction. By means of the two helical toothings, this axial displacement causes the timing gear to be rotated relative to the output part and therefore the camshaft to be rotated relative to the crankshaft.
In a rotary piston adjuster, the timing gear is connected in a rotationally fixed manner to a stator. The stator and the output part are arranged concentrically with each other. The radial intermediate space between these two components accommodates at least one, but generally a number of, cavities which are spaced apart in the circumferential direction. The cavities are bounded in a pressure tight manner in the axial direction by means of side walls. A vane connected to the output part extends into each of these cavities. This vane divides each cavity into two pressure chambers. By means of specific connection of the individual pressure chambers to a hydraulic medium pump or a hydraulic medium outlet, the phase of the camshaft can be set or maintained relative to the crankshaft.
In order to control the camshaft adjuster, sensors detect the characteristic data of the engine, such as, for example, the load state and the speed of rotation. These data are supplied to an electronic control unit which, after comparison of the data with data on the performance characteristics of the internal combustion engine, controls the adjusting motor of the camshaft adjuster and the inflow and the outflow of hydraulic medium to/from the various pressure chambers.
The axial position of the camshaft in the cylinder head of an internal combustion engine is determined by an axial bearing acting on two sides. Ideally, this is situated at the camshaft-adjuster end of the camshaft. This avoids displacements of the control drive plane due to thermal lengthening of the camshaft under operating conditions.
An axial bearing of this type is disclosed, for example, in DE 199 58 629 A1. In this case, the axial bearing comprises an encircling radial web which is designed integrally with the camshaft and engages in an annularly encircling groove of a bearing shell. This design of an axial mounting of the camshaft is not suitable in the case of use of a camshaft adjuster with a central valve which is controlled by a central magnet, since large tolerances result due to the interaction of various components between the camshaft axial bearing and central magnet. A central magnet having a large stroke is therefore required, as a result of which the axial overall length of the camshaft adjuster is considerably enlarged.
DE 100 13 877 A1 presents a device for changing the control times of gas exchange valves of an internal combustion engine, the camshaft axial bearing formed on that side of the camshaft adjuster which faces away from the cam. A pressure medium adapter is connected by means of a fastening screw to a component of the camshaft adjuster that is fixed on the camshaft. A radially extending collar is formed on that side of the pressure medium adapter which faces away from the camshaft adjuster. In addition, a washer is arranged between the pressure medium adapter and the camshaft adjuster. The collar of the pressure medium adapter and the washer form an annularly encircling groove on the outer circumferential surface of the pressure medium adapter, into which a component secured on the cylinder head, such as, for example, the cylinder head itself, a bearing bridge or a housing part engages. As a result, the camshaft is secured against axial displacement in relation to the cylinder head.
This design of an axial mounting of a camshaft by means of a washer and a pressure medium adapter, which is fitted on that side of the camshaft adjuster which faces away from the cam, permits the use of a central valve fitted within the camshaft or the rotor of the camshaft adjuster. In this solution, the small number of components between the camshaft axial bearing and the central magnet necessary for adjusting the central valve means that the tolerance chain and therefore the stroke and therefore axial overall length of the central magnet can be reduced.
A disadvantage in this embodiment is the large number of components required for the axial mounting of the camshaft. In addition to higher costs and weight of the additional components, this results in an increased outlay on installation. In addition, installation errors, such as, for example, the inadvertent omission of the shim, are possible.